Influence of alumina precursors on the microstructural and physicochemical characteristics of alumina-coated silica nanocomposite particles

The synthesis of alumina nano-shells in core-shell structures has emerged as an acknowledged challenge owing to the diverse polycrystalline nature and complex phase behavior of alumina under different conditions. By thoroughly examining the speciation, stability, and adsorption behaviors of various alumina precursors onto the core particle surface, we propose that the particle agglomeration phenomenon observed when using inorganic aluminum salts is primarily due to the formation of amorphous aluminum hydroxides from hydrolysis and polymerization reactions, which cover the surface of silica nanoparticles and adhere to these particles through the surface precipitation mechanism. The adsorption of polymeric hydroxyl-aluminum clusters featuring an Aluminum-13 Keggin-type structure, however, occurs directly onto the particle surface through complexation, followed by in-situ hydrolysis at the surface hydroxyl sites. The alumina-coated silica nanoparticles fabricated with polynuclear hydroxyl-aluminum species have been effectively synthesized, demonstrating remarkable anti-sedimentation characteristics and long-term suspension stability, as confirmed by the transmission electron microscopy (TEM) morphological analysis and ζ-potential measurements. Furthermore, by employing a comprehensive array of characterization techniques, we have systematically investigated the variations in microstructural attributes and physicochemical properties of nanocomposite particles before and after undergoing calcination temperature of 1000 °C. Consequently, nanocomposite solid particles possessing distinct coordination structures and crystalline characteristics were successfully prepared, as determined by solid-state 27Al nuclear magnetic resonance (NMR) and X-ray powder diffraction (XRD) analyses. Our findings offer valuable insights to direct the preparation and development of alumina nano-shells.